Radioactive tracer technique for identifying formations contributing unwanted fluid to productive wells



y 2, 1967 R. L. CALDWELL 3,317,726

RADIOACTIVE TRACER TECHNIQUE FOR IDENTIFYING FORMATIONS CONTRIBUTING UNWANTED FLUID TO PRODUCTIVE WELLS Filed Aug. 29, 1963 SALT WATER SAND OILTSAND 25 SALT WATER SAND OIL SAND RICHARD L. CALDWELL INVENTOR.

BY MMLZW ATTORNEY United States Patent RADIOACTIVE TRACER TECHNIQUE FOR IDEN- TIFYIN G FORMATIONS CONTRIBUTING UN WANTED FLUID T0 PRODUCTIVE WELLS Richard L. Caldwell, Dallas, Tex., assignor to Mobil Oil Corporation, a corporation of New York Filed Aug. 29, 1963, Ser. No. 305,389 9 Claims. (Cl. 250-435) This invention relates to the identification of formations which contribute unwanted fluid to a productive well and has for an object the provision of a radioactive technique of identifying formation sources of unwanted fluid which flows into a borehole and contributes to hydrocarbon fluids produced from a productive zone traversed by the borehole.

In productive wells, leaks may occur or develop in the casing or in cement :behind the casing from a water zone to the level of a productive zone. When this occurs the water zone will contribute unwanted water to the production flowing from the well. In order to effectively stop the leak by cementing, the cement must be injected into the formation contributing the unwanted water. Thus, it is necessary to locate and identify the zone contributing the unwanted water in order to perform effective remedial operations on the well.

In accordance with the present invention, there is provided a method of identifying the Zone or zones contributing unwanted fluid. More particularly, in one aspect, the method comprises the steps of irradiating with primary radiation formations other than the productive formation to produce the activation of elements behind the casing and detecting, at a position away from the zone of irradiation, delayed secondary radiation in fluid which flows into the borehole. The detecting operation additionally is carried out in the vicinity of the point of entry of fluid into the borehole from the productive formation and in the flow path of fluid from the productive formation flowing in the borehole.

In a more particular aspect, production from the well is terminated during irradiation. After irradiation, secondary radiation emitted from fluid flowing into the borehole is detected. Each suspected zone is irradiated and tested until the zone contributing the unwanted fluid is identified by an increase in radioactivity detected in the produced fluid.

In a more particular aspect, a source of unwanted salt water is identified by irradiating with neutrons formations other than the productive formation to produce activation of sodium and chlorine in the salt water behind the casing. Delayed radiation emitted from the sodium and chlorine in the fluid flowing into the borehole is detected.

In one embodiment, intensity measurements are made of detected radiation at an energy range above 1 mev.

For further aspects and advantages of the present invention and for a more complete understanding thereof,

reference may now be had to the following detailed description taken in conjunction with the accompanying drawing which illustrates a system for carrying out the present invention.

Referring more particularly to the drawing, there is illustrated a flowing well with a borehole unit 10 positioned within a borehole 11. The borehole 11 is cased with iron casing 12 and cemented with cement 13 between the casing and the borehole wall. Production tubing is illustrated at 14, with the annulus between the tubing 14 and the casing 12 being sealed by packer arrangement illustrated at 15. The tool 10 is supported by a cable 16 which passes to the surface and is wound and unwound upon a drum 17. A motor 18 drives the drum 17 by way of mechanical connection 19 to move the in- "ice strument 10 through the borehole to locate and identify formations which may be contributing unwanted fluid to the production flowing from the well. For example, if leaks develop in the casing or between the casing 12 and the formation 20, or if the well has not been properly cemented opposite formation 20, salt water from formation 21 may flow, by the path indicated by arrow 22, to the productive zone 23. If this occurs, salt water from formation 21, along with hydrocarbon fluids from formation 23, will flow through perforations 24 into the borehole. Although the well may be completed only opposite one formation, the lower portion of the figure is illustrated to show an open-hole completion and leakage occurring in the cement or between the cement and the casing. In this illustration, leakage occurs by way of path 25 from formation 26 into the borehole. To prevent the well from producing salt water from the salt water formation or formations, the location of the source or sources of unwanted salt water must be identified whereby corrective steps can be taken effectively to seal the leaks.

To identify the source of unwanted fluid, the borehole tool 10 is provided with a radioactive source 26 spaced from a radiation detector 27. Coupled to detector 27 and employed at the surface is a recorder 28. Briefly,

" the identification and location technique comprises the steps of irradiating the suspected formation to induce radioactivity in the unwanted fluid, such as salt water. The radioactivity will travel with the salt water as the well is produced. The radiation detector detects delayed radiation emitted from the salt of the water when the Water flows into the well and past the detector. An increase in intensity as reflected by the trace recorded by the recorder 28 indicates that the irradiated zone is supplying some of the produced water while no change in intensity implies little or no water production from the suspected zone. Each suspected zone, which may be above or below the producing zone, is tested individually. Suspected zones may be located from prior logs.

To lower the borehole tool 10 into the well, the tool is inserted through a conventional wellhead assembly 29 which includes a lubricator. Background radiation first is detected by detector 27 and observed during normal production. The background measurements may be made anywhere in the well, but preferably are made with the detector 27 opposite the productive formation. To effectively carry out the measurements, the tool is provided with a shield 30 between the source and detector which may be spaced apart about 10l5 feet. If desirable, the source 26 may be removed while background radiation, as well as induced delayed radiation, is being detected. After the background radiation has been measured, the well is then shut in by closing valve 31 and positioning the logging tool with its radioactive source 26 opposite the suspected water zone. The suspected formation is irradiated with primary radiation for a time sufllcient to activate the salt in the water. Valve 31 then is opened to allow the well to start producing and measurements are made of the intensity of radiation emitted from the fluid produced. In detecting the induced radiation for the production of intensity measurements, the detector is positioned away from the bombardment site and in the embodiments illustrated preferably opposite the productive zone.

It has been found that the zone of unwanted water may be more readily identified if the zone is irradiated with neutrons to produce delayed activity in sodium and/or chlorine upon the capture of thermal neutrons. The detection of the delayed radiation emitted from these two elements is preferred since they emit energetic gamma rays and have a relatively long half life. As is well known, the sodium-24 nucleus produced by the re- 3 action Na (n, 'y)Na has a fifteen-hour half life and emits 1.39 mev. beta rays and 1.37 mev. and 2.75 mev. gamma rays. The chlorine-38 nucleus produced by the reaction Cl (n,v)Cl has a 37.5-minute half life and emits beta rays of 4.81, 2.77, and 1.11 mev. and gamma rays of 2.15 and 1.60 mev.

The duration of irradiation depends among other things upon the strength of the neutron source. For example, with a conventional plutonium-beryllium source the suspected formation may be irradiated for a duration of about five hours to produce adequate activity. If the suspected formation is relatively close to the productive formation, the irradiation may be carried out for a lesser time period. The contribution of sodium and chlorine to the measurements recorded depends among other factors upon the time required for the irradiated fluid to flow from the irradiated zone to the productive zone. For example, if the flow time is of the order of several hours, delayed gamma radiation from sodium will be the predominant radiation detected. Preferably, intensity measurements are made of the radiation detected above 1 mev. to measure the intensity of delayed gamma rays emitted by sodium and chlorine respectively at 1.37, 2.75 mev. and at 1.60, 2.15 mev. To carry out the selective measurements, the detector 27 employed may comprise a scintillation crystal 32 coupled to a photomultiplier tube 33. As is well known, the photomultiplier tube 33 produces electrical pulses having a height proportional to the energy of radiation detected. The output of the tube 33 is applied to the surface by way of conductor 34, amplifier 35, and conductor 36, which passes through cable 16. At the surface, signals from conductor 36 are taken by way of slip ring and brush arrangement 37 and 38 and applied by way of amplifier 39 to an energy discriminator 40. This discriminator may be an integral discriminator or one capable of discriminating between two energy levels. The latter type, of which is illustrated, may be adjusted by adjustment of lower threshold control 41 and window width control 42 to be responsive to the radiation detected at an energy range above 1 mev. The output of the discriminator is applied to count rate meter 43 and then to recorder 28, the chart of which is driven with respect to time. Intensity increases in the direction of the arrow 44. As illustrated at 45, the trace produced will reflect an increase in intensity if the formation irradiated is producing the salt water. Intensity measurements of the background radiation are also made, as described above, at an energy range of above 1 mev. and subtracted from the intensity measurements made of the induced radiation.

The sensitivity of the method increases with decreasing background radiation. Therefore, the contribution to background radiation by the neutron source should be kept as low as possible. This contribution can be reduced by increasing the source-detector spacing. A large spacing is required to keep the detector from counting radiation from the neutron source and radioactivity produced in the tubing, casing, cement, and formation. As an alternative, as mentioned above, the source may be removed when the initial background count is taken and when the induced radioactivity is detected.

While a scintillation counter, photomultiplier tube, and and energy discriminator are disclosed in the system, a Geiger counter or ionization chamber may be employed instead to measure the total intensity of radiation detected.

It is to be understood that the present invention is applicable to wells which produce hydrocarbon fluids as a result of the natural reservoir force or wells which are made to produce by artificial means, such as pumping. In the latter case, the packer is removed and the borehole tool lowered into the borehole through the annulus between the production tubing 14 and the casing 12.

In one embodiment, the analyzer, or discriminator 40 was of the type manufactured by Baird-Atomic, Cambridge, Mass., Model 510.

Having described the invention, it will be understood that modifications may suggest themselves to those skilled in the art, and it is intended to cover all those that fall within the scope of the appended claims.

What is claimed is:

1. The method of identifying a formation source of salt water which flows into a cased borehole and contributes to hydrocarbon fluids produced from said borehole by a productive formation traversed by said borehole and located vertically from said source, comprising the steps of irradiating with neutrons formations other than said productive formation to produce activation of elements present in water behind said casing, said elements being productive of delayed gamma radiation upon the irradiation thereof with neutrons, and at a position away from the zone of irradiation detecting gamma radiation emitted from elements .in said fluid flowing into said borehole to detect for the presence of delayed gamma radiation emitted from elements in fluid irradiated with neutrons, said detecting operation being carried out in the vicinity ofthe point of entry of fluid into said borehole from said productive formation and in the flow path of fluid from said productive formation flowing in said borehole.

2. The method of claim 1 comprising the step of measuring the intensity of delayed gamma radiation detected at an energy range above 1 mev.

3. The method of identifying a salt-water formation from which salt water flows into a cased borehole at the level of a productive formation zone located vertically from said salt-water formation and contributes to hydrocarbon fluids produced from said productive formation comprising investigating each suspected formation separately by carrying out the steps of terminating the flow of fluid from said borehole, irradiating with neutrons for a predetermined period of time a suspected formation other than said productive formation to produce activation of sodium and chlorine in salt water behind said casing, said sodium and chlorine being productive of delayed gamma radiation upon the capture of thermal neutrons, flowing fluid from said borehole, at a position away from the zone of irradiation, in the vicinity of the point of entry of fluid into said borehole from said productive formation, and in the flow path of fluid from said productive formation flowing in said borehole, detecting radiation including radiation emitted from fluid flowing into said borehole at the level of said productive formation to detect for the presence of delayed gamma radiation emitted at least from sodium in fluid irradiated with neutrons, and measuring the intensity of radiation detected at an energy range above 1 mev.

4. The method of claim 3 wherein prior to irradiation background radiation is detected and measured for comparison with the measurements obtained after irradiation.

5. A method of identifying a source of unwanted fluid which flows into a cased borehole and contributes to hydrocarbon fluids produced from said borehole from a productive formation located vertically from said source of unwanted fluid, comprising the steps of:

irradiating with'neutrons for a predetermined time period suspected formations other than said productive formation to produce activation of elements behind said casing,

said activated elements being productive of delayed radiation upon the irradiation thereof with neutrons, and

at a position away from the zone of irradiation detecting for delayed secondary radiation emitted from activated elements in fluid flowing into said borehole,

said detecting operation being carried out in the vicinity of the point of entry of fluid into said borehole from said productive formation and in the flow path of fluid from said productive formation flowing in said borehole,

each suspected formation being irradiated with neutrons and the detection of secondary radiation being carried out after the irradiation of each formation prior to the irradiation of a subsequent formation.

6. The method of claim 5 wherein:

the secondary irradiation detected is gamma radiation.

7. The method of claim 6 wherein:

prior to each irradiation period the flow of fluid from the borehole is terminated and after each irradiation period during the detection of gamma radiation, fl-uid is allowed to flow from said borehole.

8. The method of claim 7 wherein:

said detecting operation is carried out substantially at the level of the point of entry of fluid into said borehole from said productive formation.

9. The method of identifying a source of unwanted fluid which flows into a borehole and contributes to hydrocarbon fluid produced from a productive formation traversed by said borehole comprising the steps of:

terminating the flow of fluid from said borehole,

irradiating with primary radiation a formation other than said productive formation,

flowing fluid from said borehole, and

at a position away from the zone of irradiation and away from the influence of secondary radiation immediately emitted from elements following irradiation, detecting for irradiation emitted from fluid flowing into said borehole,

said detecting operation being carried out in the vicinity of the point of entry of fluid into said borehole from said productive formation and in the flow path of fluid from said productive formation flowing in said borehole.

References Cited by the Examiner UNITED STATES PATENTS 2,289,926 3/1940 Neulfeld 250-836 2,335,409 8/1941 Hare 250-83.6 2,644,891 3/1950 Herzog 250-83.6 2,747,099 1/1953 Nowak -25043.5 2,943,197 10/1954 De Witte 25083.6 2,932,741 4/1960 McKay 25043.5 3,115,576 12/1963 Rickard 250106 X 3,129,331 4/1964 B-ourne et a1. 250-106 X 25 RALPH G. NILSON, Primary Examiner.

S. ELBAUM, Assistant Examiner. 

9. THE METHOD OF IDENTIFYING A SOURCE OF UNWANTED FLUID WHICH FLOWS INTO A BOREHOLE AND CONTRIBUTES TO HYDROCARBON FLUID PRODUCED FROM A PRODUCTIVE FORMATION TRAVERSED BY SAID BOREHOLE COMPRISING THE STEPS OF: TERMINATING THE FLOW OF FLUID FROM SAID BOREHOLE, IRRADIATING WITH PRIMARY RADIATION A FORMATION OTHER THAN SAID PRODUCTIVE FORMATION, FLOWING FLUID FROM SAID BOREHOLE, AND AT A POSITION AWAY FROM THE ZONE OF IRRADIATION AND AWAY FROM THE INFLUENCE OF SECONDARY RADIATION IMMEDIATELY EMITTED FROM ELEMENTS FOLLOWING IRRADIATION, DETECTING FOR IRRADIATION EMITTED FROM FLUID FLOWING INTO SAID BOREHOLE, SAID DETECTING OPERATION BEING CARRIED OUT IN THE VICINITY OF THE POINT OF ENTRY OF FLUID INTO SAID BOREHOLE FROM SAID PRODUCTIVE FORMATION AND IN THE FLOW PATH OF FLUID FROM SAID PRODUCTIVE FORMATION FLOWING IN SAID BOREHOLE. 